1. Technical Field
This invention relates to an ash melting furnace whose main duty is to melt away the ash that has resulted from incineration disposal of municipal and other sorts of wastes.
2. Background Art
Conventionally, most of the wastes such as municipal and industrial are disposed by incineration, and combustion ashes resulted therefrom are buried in the ground. Today, however, this practice has run into two major difficulties: for one, the land to be filled with such ashes is getting scarce, and for the other, the environmental regulations applicable to both incineration and ash-burying are becoming more stringent.
It is for these reasons that methods of making slag out of the ash by melting it at a high temperature and appropriately solidifying are drawing attention to the prospects of achieving volume reduction, rendering the ash non-polluting, and reusing it as a new resource. Thus, there are already a number of plants operating on an ash melting method of one sort or another.
However, the reality is that most of them are suffering from high operation costs because they rely on electricity or oil as the heat source for ash melting: this is one significant factor that is preventing this technology from spreading widely.
To solve this difficulty, a process has been proposed
wherein the waste is incinerated in such a way as to leave enough unburnt matter (e.g.,unburnt carbon) in the ash and the unburnt carbon is burned upon supplement of combustion air so as to use it as the major fuel (or major heat source) in the ash melting (described in Japanese Patent Application, Second Publication No. 4-18206). An ash melting furnace operating on this principal has demonstrated that a remarkable reduction of operating cost is indeed possible. Thus, this has reduced the necessary amount of electricity and oil used as the main heat source.
However, there remains another problem: because of the variable moisture content and changing quality of the carbonaceous substances in the waste, a property which is rather natural with municipal waste, there can be cases where ash melting becomes imperfect even though the content of the unburnt matter (called "unburnt carbon" hereinafter) is controlled to a prescribed value or range. To counteract this sort of variation, a heating unit, e.g., electric heater, may be provided in the furnace hearth.
Yet unstable melting may take place if such a quality variation occurs too rapidly, or when the combustion in the main incinerator is unsteady. For this contingency, the use of an oil burner or a plasma torch that many ash melting furnaces utilize as the main heat source for ash melting (see, for example, Japanese Patent Application, Laid-Open Publication No. 1-270990) as auxiliary heating means may be a workable solution, but then it must be done without incurring an excessive rise in the system operation cost.